The invention relates to a method to form a plurality of conductive pathways inside a through hole disposed on a printed circuit board.
Referring to FIG. 1, multilayer circuit substrate 10 is formed from individual substrates 12, 14, and 16 which are separated by insulating layers 18 and 20. Interlayer interconnections on printed circuit board patterns are made by via holes when the holes are in the same substrate. For example, via hole 60 in substrate 12 connects the circuitry on surface 22 with circuitry on surface 24. Connections between conductive patterned layers on different substrates are made by through (the layers) holes. A single through hole can make connections between a number of patterns on different layers. Plated through hole 40 electrically connects conductor 42 on surface 22 with conductor 44 on surface 26, conductor 46 on surface 26, and conductor 48 on surface 32.
In order to insulate circuitry on surfaces 28 and 30 from through hole 40, guard bands 50 and 52 are disposed around plated through hole 40 on surfaces 28 and 30, respectively. These guard bands allow for the imperfect positioning and mis-alignment of the hole relative to the conducting patterns on surfaces 28 and 30. Guard bands 50 and 52 are formed from a dielectric material. Referring to FIG. 2, a top view of substrate surface 60 shows a plurality of conductors 64 each of which are electrically insulated from surface 60 by a plurality of guard bands 62.
Using prior art technology, multiple sets of inter-layer interconnections require multiple through holes. These multiple through holes and the associated guard bands can require a large amount of space on a crowded circuit board. However, for high resolution (high density) printed wiring boards the through holes must be small. For a high density board the spacing and the hole sizes become very small.
The present invention comprises a method for making segmented through holes in a printed circuit board. A mixture of a printing ink and magnetic particles is applied to the walls of a through hole disposed in a multilayer circuit board. The circuit board is then subjected to an energy field to form the ink into a segmented pattern on the walls of the through hole.
When using a conductive ink, the segmented pattern formed on the walls of the through hole comprises a plurality of electrically conductive pathways. When using a plating resist ink, the patterned through hole is then plated to form a plurality of electrically conductive pathways. When using an etch resist ink, the through hole is plated prior to application of, and segmentation of, the ink/magnetic particle mixture. After forming the segmented pattern on the walls of the through hole, the through hole is then etched to form a plurality of electrically conductive pathways.